High-filtration efficiency wall-flow filter

ABSTRACT

The invention relates to a method for producing a wall-flow filter for removing fine particulate solids from gases, and to the use thereof for cleaning exhaust gases of an internal combustion engine. The invention also relates to a correspondingly produced exhaust-gas filter having a high filtration efficiency.

The present invention relates to a method for producing a wall-flowfilter for removing fine particulate solids from gases, and to the usethereof for cleaning exhaust gases of an internal combustion engine. Theinvention also relates to a correspondingly produced exhaust-gas filterhaving a high filtration efficiency.

The exhaust gas of internal combustion engines in motor vehiclestypically contains the harmful gases carbon monoxide (CO) andhydrocarbons (HC), nitrogen oxides (NO_(x)), and possibly sulfur oxides(SO_(x)), as well as particulates that mostly consist of solidcarbon-containing particles and possibly adherent organic agglomerates.These are called primary emissions. CO, HC, and particulates are theproducts of the incomplete combustion of the fuel inside the combustionchamber of the engine. Nitrogen oxides form in the cylinder fromnitrogen and oxygen in the intake air when combustion temperaturesexceed 1200° C. Sulfur oxides result from the combustion of organicsulfur compounds, small amounts of which are always present innon-synthetic fuels. Compliance in the future with statutory exhaustemission limits for motor vehicles applicable in Europe, China, NorthAmerica, and India requires the extensive removal of said harmfulsubstances from the exhaust gas. For the removal of these emissions,which are harmful to health and environment, from the exhaust gases ofmotor vehicles, a variety of catalytic technologies for the purificationof exhaust gases have been developed, the fundamental principle of whichis usually based upon guiding the exhaust gas that needs purificationover a flow-through or wall-flow honeycomb body with a catalyticallyactive coating applied thereto. The catalytic converter facilitates thechemical reaction of different exhaust gas components, formingnon-hazardous products, such as carbon dioxide, water, and nitrogen.

The flow-through or wall-flow honeycomb bodies just described are alsocalled catalyst supports, carriers, or substrate monoliths, as theycarry the catalytically active coating on their surface or in the wallsforming this surface. The catalytically active coating is often appliedto the catalyst support in the form of a suspension in a so-calledcoating operation. Many such processes in this respect were published inthe past by automotive exhaust-gas catalytic converter manufacturers(EP1064094B1, EP2521618B1, WO10015573A2, EP1136462B1, U.S. Pat. No.6,478,874 B1, U.S. Pat. No. 4,609,563A, WO9947260A1, JP5378659B2,EP2415522A1, JP2014205108A2).

The operating mode of the internal combustion engine is decisive for thepossible methods of harmful substance conversion in the catalyticconverter in each case. Diesel engines are usually operated with excessair, most spark-ignition engines with a stoichiometric mixture of intakeair and fuel. “Stoichiometric” means that on average exactly as much airis available for combustion of the fuel present in the cylinder as isrequired for complete combustion. The combustion air ratio A (A/F ratio;air/fuel ratio) sets the air mass m_(L,actual) which is actuallyavailable for combustion in relation to the stoichiometric air massm_(L,st):

$\lambda = \frac{m_{L,{actual}}}{m_{L,{st}}}$

If λ<1 (e.g., 0.9), this means “air deficiency” and one speaks of a richexhaust gas mixture; λ>1 (e.g., 1.1) means “excess air” and the exhaustgas mixture is referred to as lean. The statement λ=1.1 means that 10%more air is present than would be required for the stoichiometricreaction.

When lean-burn motor vehicle engines are mentioned in the present text,reference is thereby made mainly to diesel engines and to predominantlyon average lean-burn spark-ignition engines. The latter are gasolineengines predominantly operating on average with a lean A/F ratio(air/fuel ratio). In contrast, most gasoline engines are operated withan on average stoichiometric combustion mixture. In this respect, theexpression “on average” takes into consideration the fact that moderngasoline engines are not statically operated with a fixed air/fuel ratio(A/F ratio; λ value). It is rather the case that a mixture with adiscontinuous course of the air ratio λ around λ=1.0 is predetermined bythe engine control system, resulting in a periodic change betweenoxidizing and reducing exhaust gas conditions. This change in the airratio λ is significant for the exhaust gas purification result. To thisend, the λ value of the exhaust gas is regulated with a very short cycletime (approx. 0.5 to 5 Hz) and an amplitude Δλ of 0.005≤Δλ≤0.07 aroundthe value λ=1.0. On average, the exhaust gas under such operating statesshould therefore be described as “on average” stoichiometric. In orderto ensure that these deviations do not adversely affect the result ofexhaust gas purification when the exhaust gas flows over the three-waycatalytic converter, the oxygen-storing materials contained in thethree-way catalytic converter balance out these deviations by absorbingoxygen from the exhaust gas or releasing it into the exhaust gas asneeded (R. Heck et al., Catalytic Air Pollution Control, CommercialTechnology, Wiley, 2nd edition 2002, p. 87). However, due to the dynamicmode of operation of the engine in the vehicle, further deviations fromthis state also occur at times. For example, under extreme accelerationor while coasting, the operating states of the engine, and thus of theexhaust gas, can be adjusted and can, on average, be hypostoichiometricor hyperstoichiometric. However, lean-burn spark-ignition engines havean exhaust gas which is predominantly, i.e., for the majority of theduration of the combustion operation, combusted with an air/fuel ratiothat is lean on average.

The harmful gases carbon monoxide and hydrocarbons from a lean exhaustgas can easily be rendered harmless by oxidation on a suitable oxidationcatalyst. In a stoichiometrically operated internal combustion engine,all three harmful gases (HC, CO, and NOx) can be eliminated via athree-way catalytic converter.

The reduction of nitrogen oxides to nitrogen (“denitrification” of theexhaust gas) is more difficult on account of the high oxygen content ofa lean-burn engine. A known method is selective catalytic reduction(SCR) of the nitrogen oxides in a suitable catalytic converter or SCRcatalytic converter for short. This method is currently preferred forthe denitrification of lean-engine exhaust gases. The nitrogen oxidescontained in the exhaust gas are reduced in the SCR method with the aidof a reducing agent metered into the exhaust tract from an externalsource. Ammonia is used as the reducing agent, which converts intonitrogen and water the nitrogen oxides present in the exhaust gas at theSCR catalytic converter. The ammonia used as reducing agent may be madeavailable by metering an ammonia precursor compound, for example urea,ammonium carbamate, or ammonium formate, into the exhaust tract, and bysubsequent hydrolysis.

Diesel particulate filters (DPF) or gasoline particulate filters (GPF)with and without additional catalytically active coating are suitableaggregates for removing the particulate emissions. In order to meet thelegal standards, it is desirable for current and future applications forthe exhaust gas aftertreatment of internal combustion engines to combineparticulate filters—particularly those of the wall-flow type—with othercatalytically active functionalities, not only for reasons of cost butalso for reasons of installation space. The use of a particulate filter,whether catalytically coated or not, leads to a noticeable increase inthe exhaust-gas back pressure in comparison with a flow-through supportof the same dimensions and thus to a reduction in the torque of theengine or possibly to increased fuel consumption. In order to notincrease the exhaust-gas back pressure even further, the amounts ofoxidic support materials for the catalytically active noble metals ofthe catalytic converter or oxidic catalyst materials are generallyapplied in smaller quantities in the case of a filter than in the caseof a flow-through support. As a result, the catalytic effectiveness of acatalytically coated particulate filter is frequently inferior to thatof a flow-through monolith of the same dimensions.

There have already been some efforts to provide particulate filterswhich have good catalytic activity due to an active coating and yet havethe lowest possible exhaust-gas back pressure. On the one hand, it hasproven to be advantageous if the catalytically active coating is notpresent as a layer on the wall of a porous wall-flow filter, but insteadthe wall of the filter is interspersed with the catalytically activematerial (WO2005016497A1, JPH01-151706, EP1789190B1). For this purpose,the particle size of the catalytic coating is selected such that theparticles penetrate into the pores of the wall-flow filters and can befixed there by calcination.

A further functionality of the filter which can be improved by a coatingis its filtration efficiency, i.e., the filtering effect itself. Theincrease in the filtration efficiency of catalytically inactive filtersis described in WO2012030534A1. In this case, a filtration layer(“discriminating layer”) is created on the walls of the flow channels ofthe inlet side by the deposition of ceramic particles via a particleaerosol. The layers consist of oxides of zirconium, aluminum, orsilicon, preferably in fiber form ranging from 1 nm to 5 μm, and have alayer thickness greater than 10 μm, typically 25 μm to 75 μm. After thecoating process, the applied powder particles are calcined in a thermalprocess.

A coating inside the pores of a wall-flow filter unit by spraying dryparticles is described in U.S. Pat. No. 8,388,721 B2. In this case,however, the powder should penetrate deeply into the pores. 20% to 60%of the surface of the wall should remain accessible to soot particles,thus open. Depending on the flow velocity of the powder/gas mixture, amore or less steep powder gradient between the inlet and outlet sidescan be set.

The introduction of the powder into the pores, for example by means ofan aerosol generator, is also described in EP2727640A1. Here, anon-catalytically coated wall-flow filter is coated using a gas streamcontaining, for example, aluminum oxide particles in such a way that thecomplete particles, which have a particle size of 0.1 μm to 5 μm, aredeposited as a porous filling in the pores of the wall-flow filter. Theparticles themselves can realize a further functionality of the filterin addition to the filtering effect. For example, these particles aredeposited in the pores of the filter in an amount greater than 80 g/lbased on the filter volume. They fill in 10% to 50% of the volume of thefilled pores in the channel walls. This filter, both loaded with sootand without soot, has an improved filtration efficiency compared withthe untreated filter together with a lower exhaust-gas back pressure ofthe soot-loaded filter.

EP2502661A1 and EP2502662B1 mention further methods for the on-wallcoating of filters by powder application. Corresponding apparatuses forapplying a powder/gas aerosol to the filter, in which the powderapplicator and the wall-flow filter are each separated so that air issucked in through this space during coating, are also shown there. Afurther method in which a membrane (“trapping layer”) is produced on thesurfaces of the inlet channels of filters in order to increase thefiltration efficiency of catalytically inactive wall-flow filters isdescribed in patent specification U.S. Pat. No. 8,277,880B2. Thefiltration membrane on the surfaces of the inlet channels is produced bysucking a gas stream loaded with ceramic particles (for example, siliconcarbide, cordierite) through. After application of the filter layer, thehoneycomb body is fired at temperatures greater than 1000° C. in orderto increase the adhesive strength of the powder layer on the channelwalls.

WO2011151711A1 describes a method by which a dry aerosol is applied toan uncoated or catalytically coated filter. The aerosol is provided bythe distribution of a powdered high-melting metal oxide having anaverage particle size of 0.2 μm to 5 μm and guided through the inletside of a wall-flow filter by means of a gas stream. In this case, theindividual particles agglomerate to form a bridged network of particlesand are deposited as a layer on the surface of the individual inletchannels passing through the wall-flow filter. The typical powderloading of a filter is between 5 g and 50 g per liter of filter volume.It is expressly pointed out that it is not desirable to obtain a coatinginside the pores of the wall-flow filter with the metal oxide.

EP1576998A2 describes the production of a thin membrane, <5 μm, on theoutput side of the porous cell wall. The porous membrane is made ofnanoparticles having diameters between 20 and 200 nm. In order to fixthe membrane on the outlet side of the wall-flow filter, a finalcalcination takes place here.

U.S. Pat. No. 9,745,227B2 describes the production of an applicationlayer with porous particle agglomerates having a diameter between 10 and200 μm. These agglomerates, in turn, are prepared in an upstream processfrom particles having dimensions between 0.01 and 5 μm. The appliedlayer must then be calcined.

WO18115900A1 mentions the oxidic powders of synthetic ash with a d90<1μm. The filters are coated therewith in such a way that a packed bed ofsynthetic ash is formed on the filter walls.

However, there are further requirements for particulate filters forwhich solutions are still being sought. This relates, for example, toimproved soot burn-off and the introduction of additional reactive zonesfor controlling the catalytic reactions.

The object of the present invention was accordingly to specify furtherand improved particulate filters, also called wall-flow filters, whichare optimized with regard to their filtration efficiency and theresulting exhaust-gas back pressure. Moreover, the filters should beeasy to manufacture in a robust and flexible working process andinexpensive.

These and other objects that are obvious from the prior art to a personskilled in the art are achieved by a method according to independentclaim 1. Preferred embodiments of the method can be found in thesubclaims that are dependent upon claim 1. Claim 10 is directed to acorrespondingly produced wall-flow filter. Claim 11 includes a preferreduse.

In a method for producing a wall-flow filter for purifying gases fromsmall particulate solids, the stated object can be achieved extremelyadvantageously by applying a dry powder/gas aerosol to the inlet regionof the dry filter, the powder having a pyrogenic, high-melting metalcompound produced by flame hydrolysis or flame oxidation from a metalprecursor in a flame and the amount of pyrogenic high-melting compoundin the filter being less than 5 g/l. The wall-flow filters producedaccording to the invention are preferably used in the purification ofexhaust gases from internal combustion engines. They filter the sootformed during the combustion out of the exhaust gas and thereby leadonly to a slight increase in the exhaust-gas back pressure. This resultcan be achieved even with a relatively small amount of pyrogenicmaterial. Against the background of the known prior art, this was not tobe expected.

The application of the dry powder/gas aerosol to the dry, possiblycatalytically coated filter results in the powdered, pyrogenic metalcompounds being deposited following the flow of the gas on the surfaceof the input side of the filter and optionally in the pores of thefilter. In principle, the person skilled in the art knows how to producean aerosol from a pyrogenic powder and a gas in order to then guide theaerosol through the filter to which the powder is to be applied. Inorder to produce a wall-flow filter, a dry filter is advantageouslyprovided on its input surface with the dry, pyrogenic powder/gasaerosol, by dispersing the powder in a gas, directing it into a gasstream and sucking or pressing it into the inlet side of the filterwithout further supply of a gas. For reasons of occupational safety,suction is preferable to pressing.

The aerosol consisting of the gas and the pyrogenic metal compound maybe produced in accordance with the requirements of the person skilled inthe art. For this purpose, a pyrogenically produced metal powder isusually mixed with a gas(http://www.tsi.com/Aerosolgeneratoren-und-dispergierer/;https://www.palas.de/de/product/aerosolgeneratorssolidparticles). Thismixture of gas and powder produced in this way is then advantageouslyfed into the inlet side of the wall-flow filter via a gas stream. Theterm “inlet side” refers to the part of the filter formed by the inflowchannels/input channels. The input surface is formed by the wallsurfaces of the inflow channels/input channels on the input side of thewall-flow filter. The same applies mutatis mutandis to the outlet side.

All gases considered by the person skilled in the art for the presentpurpose can be used as gases for producing the aerosol and for inputtinginto the filter. The use of air is most particularly preferred. However,it is also possible to use other reaction gases which can develop eitheran oxidizing (e.g. O₂, NO₂) or a reducing (e.g. H2) activity withrespect to the powder used. With certain powders, the use of inert gases(e.g. N2) or noble gases (e.g. He) may also prove advantageous. Mixturesof the listed gases are also conceivable.

In order to be able to deposit the powder sufficiently well on and/or inthe surface of the filter wall on the inlet side of the filter, acertain suction or pressure force is needed. In orientation experimentsfor the respective filter and the respective powder, the person skilledin the art can form an idea for himself in this respect. It has beenfound that the aerosol (powder/gas mixture) is preferably sucked throughthe filter at a velocity of 5 m/s to 60 m/s, more preferably 10 m/s to50 m/s, and very particularly preferably 15 m/s to 40 m/s, since thiscorresponds to the later exhaust gas velocities. This likewise achievesan advantageous adhesion of the applied powder. Most preferably, gasvelocities corresponding to those of the later exhaust gas to bepurified are used. The use of certain gas velocities in front of thefilter during the coating in the region of the later exhaust gasvelocities in front of the filter results in a powder distribution andthus a distribution of the filtering surface in the filter whichoptimally matches the flow of particulates in the exhaust gas. Thevelocities are adapted to the requirements in the respective applicationof the filter within the scope of the invention. For filters with auniform permeability distribution over the filter, this means that, as arule, more powder is deposited in the last third of the filter in theflow direction. If permeability is distributed non-uniformly in thefilter, for example as a result of zone coatings, this usually meansthat the powder is preferably deposited in the regions of highpermeability, since the volume flow during the coating is also thehighest in the region of high permeability subsequently during use as anexhaust-gas filter.

The amount of powder in the filter depends on the type of powder and tothe dimensions of the filter and can be determined by the person skilledin the art in preliminary experiments under the given boundaryconditions (not too high an exhaust-gas back pressure). As a rule andaccording to the invention, the loading of the filter with the pyrogenicpowder is less than 5 g/l relative to the filter volume. The value ispreferably not more than 3 g/l, very particularly preferably not morethan 2 g/l. A lower limit is naturally formed by the desired increase infiltration efficiency. In this context it is particularly preferred ifthe amount of powder remaining in the filter is below 2 g/l.

By applying the special powder/gas aerosol to the filter, the filtrationefficiency or the exhaust-gas back pressure of the filter with possiblyadditional catalytic functions can be adjusted well to the respectiveconditions in the exhaust tract, for example of an automobile. Thedevelopment of the designs adapted to the different requirements in eachcase is cost-effective and flexible. The production process is alsocost-effective, since several different types of powder, whichoptionally perform different functions, can be loaded in the sameinstallation, for example with two and more applicators, directly oneafter the other in time, separated only by fractions of seconds.

The filters described herein, which are possibly previouslycatalytically coated and then loaded with pyrogenic powders, differ fromthose that are produced in the exhaust tract of a vehicle by ashdeposition during operation. According to the invention, the optionallycatalytically active filters are deliberately dusted with a specific,dry powder in the form of a pyrogenically produced material, for examplea metal oxide. As a result, the balance between filtration efficiencyand exhaust-gas back pressure can be adjusted selectively right from thestart. Wall-flow filters in which more or less undefined ash depositshave resulted from combustion of fuel, for example in the cylinderduring driving operation or by means of a burner, are therefore notincluded in the present invention.

All ceramic materials customary in the prior art can be used aswall-flow monoliths or wall-flow filters. Porous wall-flow filtersubstrates made of cordierite, silicon carbide, or aluminum titanate arepreferably used. These wall-flow filter substrates have inflow andoutflow channels, wherein the respective downstream ends of the inflowchannels and the upstream ends of the outflow channels are offsetagainst each other and closed off with gas-tight “plugs.” In this case,the exhaust gas that is to be purified and that flows through the filtersubstrate is forced to pass through the porous wall between the inflowchannel and outflow channel, which delivers an excellent particulatefiltering effect. The filtration property for particulates can bedesigned by means of porosity, pore/radii distribution, and thickness ofthe wall. The porosity of the uncoated wall-flow filters is typicallymore than 40%, generally from 40% to 75%, particularly from 50% to 70%[measured according to DIN 66133, latest version on the date ofapplication]. The average pore size (average pore diameter; d50) of theuncoated filters is at least 7 μm, for example from 7 μm to 34 μm,preferably more than 10 μm, in particular more preferably from 10 μm to25 μm, or very preferably from 15 μm to 20 μm [measured according to DIN66134, latest version on the date of application]. The completed filtershaving a pore size (d50) of typically 10 μm to 20 μm and a porosity of50% to 65% are particularly preferred.

In general and according to the invention, pyrogenically produced metalpowders are understood to be those obtained by flame hydrolysis or flameoxidation from a metal precursor in a flame and having properties suchas are described for flame-synthesized particulate products in thefollowing references, Gutsch A. et al. (2002) KONA (No. 20); Li S. etal. (2016) Progress in Energy and Combustion Science (55); Ulrich G.(1971) Combustion Science and Technology (vol. 4). Such processes havealready been established industrially since 1944, for example, atDegussa AG. The first patents for this originate from the years 1931 to1954 (U.S. Pat. Nos. 1,967,235A, 2,488,440A, DE948415C, DE 952891C).Pyrogenic silica(https://de.wikipedia.org/w/index.php?title=Pyrogenes_Siliciumdioxid&oldid=182147815) is sold, for example, by Evonik under the name Aerosil® orpyrogenic aluminum oxide under the name Aeroxide®(https://www.aerosil.com/product/aerosil/downloads/ti-1331-aerosil-and-aeroxide-for-glossy-photo-inkjet-media-en.pdf).In general, this method makes it possible to produce high-surfacecompounds, in particular oxides of various metals with a very low tampeddensity of <100 kg/m³, preferably <80 kg/m³ and very preferably <60kg/m³ (measured according to standard DIN EN ISO 787-11—latest versionon the date of application), which are preferably used according to theinvention. The porosity of these pyrogenic materials, for example of themetal oxides, is >90%, preferably >93% and very preferably >95%. This isdetermined from the ratio of the tamped density to the primary particledensity or the true non-porous density of the respective oxides. Theformula for this is:

Porosity=1−tamped density/true density

With the example of aluminum oxide, the true density(https://de.wikipedia.org/w/index.php?title=Reindichte&oldid=164022376)is 3200 to 3600 kg/m³, and the tamped density of the pyrogenic oxides isonly about 50 kg/m³. Thus, 1 m³ of powder contains only about 1.5%aluminum oxide. These are advantageously, for example, those selectedfrom the group consisting of silicon dioxide, aluminum oxide, titaniumdioxide, zirconium dioxide, cerium oxide, iron oxide, zinc oxide ortheir mixed oxides or mixtures thereof. However, it is also possible toproduce other pyrogenic oxides, mixed oxides (so-called solid solutions)or doped mixed oxides.

In flame pyrolysis, approximately spherical primary particles initiallyarise in the nanometer range (d50: 5-50 nm), which sinter togetherduring the further reaction to form highly porous chain-shapedaggregates. The aggregates usually have an average particle size (d50)of <0.5 μm and can then conglobate to form agglomerates having anaverage particle size of 10-100 μm (FIG. 1,(https://de.wikipedia.org/wiki/Pyrogenes_Siliciumdioxid)). In contrastto the agglomerates, which as a rule can be separated relatively easilyinto the aggregates by introducing energy, the aggregates are decomposedfurther into the primary particles only by intensive introduction ofenergy (Manuel Gensch, dissertation, Mechanische Stabilität vonNanopartikel-Agglomeraten bei mechanischen Belastungen, ISBN:978-3-8440-6110-9, Shaker Verlag). In the present case, the particlesizes are measured off-line by means of laser diffraction according tothe standard ISO 13320 (latest version on the date of application). If ad50 value for particles is referred to in the present text, this meansthe d50 value of a Q3 distribution.

According to the invention, a method is therefore preferred in which thepyrogenic high-melting compound is exposed to a fluid-dynamic stress(shear force) before the application to the filter. Depending on theintensity of the shear force, it can therefore be achieved in thismodification that the powder is deagglomerated or/or deaggregated. Thepowder can thus be deposited either only on the wall of the wall-flowfilter, on and in its wall, or only in the wall. As a result, thewall-flow filter can be adapted well to the underlying purificationproblem (for example filtering of small particulate or largerparticulate soots from the exhaust gas of an internal combustionengine). The strength of the shear force can be determined frompreliminary tests. The lower limit of the shear force will be foundwhere the agglomerates of the pyrogenic compound can be split intosmaller compartments. An upper limit in this respect will be formed bythe division of the aggregates into smaller units as far as the primaryparticles.

The shearing force exerted on the powder may be caused by means known tothe person skilled in the art. In order to also get more of the powders,for example, into the pores of the filter, at least some particlediameters of the powder should be smaller than the pore diameters of thewall-flow filter. The particle diameters of the agglomerates can bereduced by the milling steps for oxidic powders known to the personskilled in the art. For the pyrogenic materials, the use of which formsthe basis for this invention, methods with shearing and/or impactstresses are preferred as methods for introducing the shear force.Atomizer nozzles are included, for example, among the high shearingmethods. On the other hand, wind sifters, counter-jet mills and theimpingement on baffle plates belong to the methods which perform thebreaking up of the agglomerates of pyrogenic oxides predominantly byimpact stress. The shear force is therefore preferably produced by oneor more devices selected from the group consisting of a wear-freeatomizer nozzle, a wind sifter, a mill and a baffle plate.

The most preferred use, according to the invention, of shearing andwear-resistant atomizer nozzles in combination, for example, with abaffle plate, makes it possible to produce the required particle sizesin the process during coating (Break up and Bounce of TiO2 agglomeratesby impaction, Ihalainen, M.; Lind, T.; Arffman, A.; Torvela, T.;Jokiniemi, J. in: Aerosol Science and Technology, vol. 48, no. 1, 2014,p. 31-41; Interparticle forces in silica nanoparticle agglomerates,Seipenbusch, M. Rothenbacher, S. Kirchhoff, M. Schmid, H.-J. Kasper, G.Weber, A. P. in Journal of Nanoparticle Research; 12, 6; 2037-2044;Manuel Gensch, dissertation, Mechanische Stabilität vonNanopartikel-Agglomeraten bei mechanischen Belastungen, ISBN:978-3-8440-6110-9, Shaker Verlag; Trockene Desagglomeration vonNanopartikelagglomeraten in einer Sichtermühle, Sascha Füchsel*, KlausHusemann and Urs Peuker, Chemie Ingenieur Technik 2011, 83, no. 8,1262-1275).

In this case, the particle agglomerates are comminuted to a d50 of, forexample, approximately 2 to 8 μm, for example by the wear-resistantatomizer nozzle, fractions of a second prior to the application to thefilter. In this way, and by dilution of the aerosol with an additionaltransport gas, renewed agglomeration is avoided. If there is a need foreven finer particles, an additional baffle plate dispersion isadvantageously used. Here, too, the dispersion is carried out fractionsof a second before the coating, so that renewed agglomeration of theparticles is avoided. By means of the mass flow and the impingementspeed on the baffle plate, the particle sizes can optionally be changedcontinuously. These two dispersing methods in conjunction with the useof the pyrogenic metal compounds enable a rapidly modifiable, flexibleand cost-effective process which can be adapted to the different filterqualities or requirements.

The techniques illustrated here make it possible to adjust the particlesizes which are suitable for the present purpose very precisely. Theparticle sizes (d50) of the pyrogenic metal compounds, in particular ofthe metal oxides, are in the region of the primary particles, 1-100 nm,preferably 10-80 nm and very preferably 20-50 nm, when applied in thepores of the wall-flow filter. As a result, these particles can also bedeposited in a sufficient quantity in the superficially situated poresof the wall-flow filter. If—as stated—an incorporation of the powderinto the pores is not desired, the d50 value is rather 0.5-100, morepreferably 1-50 and very preferably 5-20 μm. Mixtures of both sizeregimes during the application, optionally staggered, are alsoconceivable. As a rule, the particle size distribution of the powderbefore the application is preferably in a range of 0.1-50 μm (d50).

The surface area of the particles in the powder is very high in thiscase. Advantageously, the pyrogenic high-melting compound has a BETsurface area of >50 m2/g, more preferably >70 m2/g and verypreferably >900 m2/g. The BET surface area is determined in accordancewith DIN ISO 9277:2003-05 (Determination of the specific surface area ofsolids by gas adsorption using the BET method). The virtually pore-freeprimary particles having a particle diameter of <100 nm even produce ahigh specific surface area of up to over 200 m²/g, which is availablefor the deposition of the nanoparticles.

Table 1 shows the dependence of the outer surface area on particle size.For the calculation, the true non-porous density of aluminum oxide wasassumed to be 3600 kg/m³ and the particle shape assumed to be a sphere.

Particle diameter (d50) [μm] 8 4 2 1 0.5 0.1 0.03* 0.01* Outer m²/g 0.20.4 0.8 1.7 3.3 16.7 55.6 166.7 surface area *Region of the pyrogenicmetal compound: primary particle diameter < 100 nm

The high outer surface area provides an excellent deposition surface forthe particles, in particular soot particles in the nanometer range. Thelow tamping density of the pyrogenic metal compound and the virtuallypore-free primary particles means an extremely high porosity availablefor the flow. Thus, the pressure rise resulting from the pyrogenicpowder on the filter is very low.

Table 2 shows, by way of example, some data of the pyrogenic oxides incomparison with customary highly porous aluminum oxides such as Puralox®from Sasol.

Table 2: Characteristic data compiled by way of example for a pyrogenicaluminum oxide produced by Evonik. The pyrogenic aluminum oxide C has aprimary particle size of 13 nm. The density of the primary particles isabout 3200 g/l. Puralox from Sasol was used here as a comparison.

Average agglomerate Average size BET primary depending surface Tampedgrain on shear area density size energy Porosity m²/g g/liter nm μmPyrogenic 98.5% about about 13 1 to 10 aluminum 100 50 oxide CCommercially about to about >1000 unmilled available highly 80% over600- 20 to 50 porous aluminum 200 1200 oxides in catalysis

In a specific embodiment of the invention, the optionally catalyticallyactivated pyrogenic high-melting compounds in the powder/gas aerosol canbe mixed with further non-pyrogenic high-melting compounds. Prior tobeing brought into the wall-flow filter, the pyrogenic metal compoundsare advantageously also mixed with other non-pyrogenic materials, e.g.oxides selected from the group consisting of silicon dioxide, aluminumoxide, titanium dioxide, zirconium dioxide, cerium oxide, iron oxide,zinc oxide or their mixed oxides or mixtures thereof, optionally withnoble metal-coated oxides or, if appropriate, ion-exchanged zeolites andmuch more. Mixed oxides with other metals, such as iron on the basis ofthe above-mentioned group, can represent catalytically interestingproperties for specific reaction sequences. As a result, the goodfiltration property and the low pressure loss caused by the pyrogenicmaterials are largely retained, while the other non-pyrogenic materialscan provide additional catalytic properties. Accordingly, it is alsopreferably possible to mix the pyrogenically produced metal oxidepowders with the other metal oxide powders and to apply them to thewall-flow filter. The individual powders each on their own or bothpowders together can have catalytic activity, for example due to noblemetal addition. This allows for an inexpensive standard process for avariety of properties of the product. The strong crosslinking of thepyrogenic compounds also leads to the fact that mixtures of pyrogenicmetal compounds with customary non-pyrogenic materials, for examplemetal oxides with a low outer surface area, lead to the same advantageswith regard to filtration efficiency and pressure loss, but the amountsof powder can be significantly reduced.

In a preferred embodiment, the filter may have been catalytically coatedprior to the application of the powder/gas aerosol. Here, catalyticcoating is understood to mean the ability to convert harmfulconstituents of the exhaust gas from internal combustion engines intoless harmful ones. The exhaust gas constituents NOx, CO, and HC andparticulate matter should be mentioned here in particular. Thiscatalytic activity is provided according to the requirements of theperson skilled in the art by a coating of the wall-flow filter with acatalytically active material. The term “coating” is accordingly to beunderstood to mean the application of catalytically active materials tothe wall-flow filter. The coating assumes the actual catalytic function.In the present case, the coating is carried out by applying acorrespondingly low-viscosity aqueous suspension, also called washcoat,or solution of the catalytically active components to the wall-flowfilter, see, for example, according to EP1789190B1. After application ofthe suspension/solution, the wall-flow filter is dried and, ifapplicable, calcined at an increased temperature. The catalyticallycoated filter preferably has a loading of 20 g/l to 200 g/l, preferably30 g/l to 150 g/l. The most suitable amount of loading of a filtercoated in the wall depends on its cell density, its wall thickness, andthe porosity. In the case of common medium-porous filters (<60%porosity) with, for example, 200 cpsi cell density and 8 mil wallthickness, the preferred loading is 20 g/l to 50 g/l (based on the outervolume of the filter substrate). Highly porous filters (>60% porosity)with, for example, 300 cpsi and 8 mil have a preferred load of 25 g/l to150 g/l, particularly preferably 50 g/l to 100 g/l.

In principle, all coatings known to the person skilled in the art forthe automotive exhaust-gas field are suitable for the present invention.The catalytic coating of the filter may preferably be selected from thegroup consisting of three-way catalyst, SCR catalyst, nitrogen oxidestorage catalyst, oxidation catalyst, soot-ignition coating. With regardto the individual catalytic activities coming into consideration andtheir explanation, reference is made to the statements inWO2011151711A1. Particularly preferably, this has a catalytically activecoating having at least one metal-ion-exchanged zeolite,cerium/zirconium mixed oxide, aluminum oxide, and palladium, rhodium, orplatinum, or combinations of these noble metals.

The powders used here can be used as such according to the invention asdescribed above. However, it is also conceivable to use dry, pyrogenicmetal compounds, in particular oxide powders, and/or non-pyrogenic metalcompounds, in particular oxides, which support a catalytic activity withregard to exhaust gas aftertreatment. Accordingly, the powder itself canlikewise be catalytically active with regard to reducing harmfulsubstances in the exhaust gas of an internal combustion engine. Suitablefor this purpose are all activities known to the person skilled in theart, such as TWC, DOC, SCR, LNT, or soot-burn-off-acceleratingcatalysts. The powder will generally have the same catalytic activity asan optionally performed catalytic coating of the filter. This furtherincreases the overall catalytic activity of the filter as compared withfilters not coated with catalytically active powder. In this respect, itmay be possible to use pyrogenic aluminum oxide, for example,impregnated with a noble metal for producing the powder/gas aerosol.Three-way activity with a coating comprising palladium and rhodium andan oxygen storage material such as cerium zirconium oxide is preferredin this context. It is likewise conceivable for catalytically activematerial to be used for the SCR reaction. Here, the powder may consist,for example, of zeolites or zeotypes exchanged with transition metalions. Very particular preference is given in this context to the use ofzeolites exchanged with iron and/or copper. CuCHA (copper-exchangedchabazite; http://europe.iza-structure.org/lZA-SC/framework.php?STC=CHA)or CuAEI (http://europe.iza-structure.org/lZA-SC/framework.php?STC=AEI)are extremely preferably used as material for producing the powder/gasaerosol. Further advantageously, an activity of the powder may consistin the improved soot combustion.

In a further embodiment, the present invention relates to a wall-flowfilter as described above. The product-characteristic advantagesdescribed above in the context of the method according to the inventionalso apply, mutatis mutandis, to the wall-flow filter according to theinvention.

The wall-flow filters according to the invention can be used in anyapplication in which finely particulate solids have to be separated fromgases. This process is advantageously used in the field of theautomobile exhaust gases which are loaded with finely distributed sootparticles from incomplete combustion. The described wall-flow filtersmay be used in an automobile exhaust system with other catalystsselected from the group consisting of TWC, SCR catalyst, nitrogen oxidestorage catalyst, diesel oxidation catalyst, and others.

The wall-flow filter produced according to the invention exhibits anexcellent filtration efficiency with only a moderate increase inexhaust-gas back pressure as compared with a wall-flow filter in thefresh state to which pyrogenic powder has not been applied. The worsethe filtration efficiency of the filter, the greater the increase infiltration efficiency resulting from the coating with pyrogenic metalcompounds. This is particularly noticeable in the case of filters whichhave been previously coated with a washcoat. The wall-flow filterpreviously coated with washcoat according to the invention preferablyexhibits a relative increase in filtration efficiency of at least 5%,preferably at least 10%, and very particularly preferably at least 20%with a relative increase in the exhaust-gas back pressure of the freshwall-flow filter of at most 40%, preferably at most 20%, and veryparticularly preferably at most 10%, as compared with a fresh filter nottreated with powder. Particularly advantageous is an improvement infiltration efficiency of at least 20% with a maximum back pressureincrease of no more than 10%.

The wall-flow filters presented here are characterized by a goodfiltration efficiency paired with a very low increase in the exhaust-gasback pressure. It is assumed that the pyrogenically produced metalcompounds after comminution only crosslink again to form largeragglomerates during deposition in the filter. They then form a loosenetwork on the wall and/or in the pores. Primarily the particle size ofthe agglomerates after the comminution described, the amount of powder,the pore diameter and the open porosity of the filter determine whetherthe pores or the cell surface or both are coated. The gases can flowthrough the extremely loose aggregate material without a large pressureloss. The large outer surface area of the pyrogenic metal compounds inturn brings about a very good filtration efficiency for the sootparticles in the nanometer range. Very small amounts of pyrogenic oxidesare sufficient to produce large outer surface areas. The oxidesconventionally used in catalysis have large internal surface areas whichare present due to a high porosity in the nanometer range, but these arenot accessible for the filtration of nanoparticles.

Dry in the sense of the present invention accordingly means exclusion ofthe application of a liquid, in particular water. In particular, theproduction of a suspension of the powder in a liquid for spraying into agas stream should be avoided. A certain moisture content may possibly betolerable both for the filter and for the powder, provided thatachieving the objective—the most finely distributed deposition of thepowder possible in or on the input surface—is not negatively affected.As a rule, the powder is free-flowing and dispersible by energy input.The moisture content of the powder or of the filter at the time ofapplication of the powder should be less than 20%, preferably less than10%, and very particularly preferably less than 5% (measured at 20° C.and normal pressure, ISO 11465, latest version on the date ofapplication).

In FIG. 1 flame pyrolysis for the production of pyrogenic silicondioxide is pictorially represented. The particles which form primarilyaggregate as a result and finally accumulate together to formagglomerates (larger chains of primary particles). File: Fumed silicaprocess.svg. (2016, November 28). Wikimedia Commons, the free mediarepository. Retrieved 08:28, Nov. 7, 2018 fromhttps://commons.wikimedia.org/w/index.php?title=File:Fumed_silica_process.svg&oldid=222460038.

FIG. 2 explains the method according to the invention and thecomminution and mixing chamber in more detail: The pyrogenic oxide 400is driven with a gas 300 under pressure under high shearing through thewear-free atomizer nozzle 700. The speed may range up to the speed ofsound. After exiting the atomizer nozzle, the gas/powder mixture 500impinges on the baffle plate 600 located in the comminution and mixingchamber 100. The gas/powder mixture thus formed with the comminutedpowder agglomerates is then mixed with the gas stream 200 and thenpasses as a diluted gas/powder mixture 800 to coat the filter. Thediluent gas 200 is required so that the inflow rate of the filter duringcoating can be varied independently of the amount of atomizer gas.

EXAMPLE 1

Coating a raw washcoat-free filter having dimensions 4.66″×6.00″ 300/8with powder.

The pyrogenically produced powder was dispersed with the aid of anatomizer nozzle at 2 bar and sucked into the filter at a rate of 20 m/s.

Relative* increase Relative* in filtration pressure efficiency increase0.6 g pyrogenic Al₂O₃/liter filter volume 5.5%   2% 1.2 g pyrogenicAl₂O₃/liter filter volume 9% 3% 0.3 g pyrogenic Al₂O₃ + 1.2 g Al₂O₃ 6%1% with a d50 of 3 μm/liter filter volume *Relative to an uncoated rawfilter substrate without additional powder coating

Example 2

In a 1st step, the filter was coated with 50 g/l washcoat in the porousfilter wall, dried and calcined. It was then coated with 2 g/lpyrogenically produced powder. The powder was dispersed at 2 bar withthe aid of a wear-free atomizer nozzle and sucked into the filter at arate of 20 m/s. The filtration efficiency increase and the increase inpressure loss were determined at 600 m³/h relative to the powder-freefilter.

Relative* increase Relative* in filtration pressure efficiency increase2 g pyrogenic Al₂O₃/liter filter volume 47% 10% *Relative to thesubstrate coated only with washcoat

1. Method for producing a wall-flow filter for purifying gases fromsmall particulate solids, wherein a dry powder/gas aerosol is applied tothe inlet region of the dry filter, characterized in that the powdercontains a pyrogenic, high-melting metal compound produced by flamehydrolysis or flame oxidation from a metal precursor in a flame, and theamount of pyrogenic high-melting compound in the filter is less than 5g/l.
 2. Method according to claim 1, characterized in that the pyrogenichigh-melting compound is selected from the group consisting of silicondioxide, aluminum oxide, titanium dioxide, zirconium dioxide, ceriumoxide, iron oxide, zinc oxide, mixed oxides of the aforementioned oxidesor mixtures thereof.
 3. Method according to claim 1, characterized inthat the pyrogenic high-melting compound is subjected to a shear forceprior to the application to the filter.
 4. Method according to claim 3,characterized in that the shear force is caused by one or more devicesselected from the group consisting of a wear-free atomizer nozzle, awind sifter, a mill and a baffle plate.
 5. Method according to claim 1,characterized in that the average particle size (d50) of the pyrogenichigh-melting compound is between 0.1 μm and 50 μm.
 6. Method accordingto claim 1, characterized in that the pyrogenic high-melting compoundhas a BET surface area of >50 m²/g.
 7. Method according to claim 1,characterized in that the pyrogenic high-melting compound in thepowder/gas aerosol is mixed with further non-pyrogenic high-meltingcompounds.
 8. Method according to claim 1, characterized in that thewall-flow filter has been catalytically coated prior to application ofthe pyrogenic high-melting compound.
 9. Method according to claim 1,characterized in that the pyrogenic and/or non-pyrogenic compoundsthemselves are catalytically active.
 10. A wall-flow filter obtainableby a method according to claim
 1. 11. A method for the purification ofautomobile gases comprising passing the automobile gases through thewall-flow filter according to claim 10.